1. Field of the Invention
The present invention is broadly concerned with crystalline bodies having atoms of Groups II and VI therein which exhibit persistent photoconductivity (PPC), a phenomenon of photo-induced conductivity that persists for a very long period of time after the removal of an exciting light source. More particularly, it is concerned with II-VI mixed crystals having the general formula Zn.sub.x Cd.sub.1-x Se, where x is up to about 0.4, and with complete electrical apparatus (e.g., sensors, detectors, and solid state imagers) using the PPC crystals of the invention.
2. Description of the Prior Art
One of the most interesting and important phenomena under intensive investigation in the field of semiconductor materials is the slow relaxation of photoexcited charge carriers, manifested as persistent photoconductivity (PPC). The PPC effect has been observed in a great variety of materials. Prior PPC materials demonstrate PPC relaxation times which generally decrease with increasing temperature. Several mechanisms have been proposed to explain the effect. One such mechanism is the so-called macroscopic barrier model, which predicts a PPC decay essentially logarithmic in time for artificially constructed layered materials. This prediction is consistent with experimental observations. However, PPC in bulk semiconductors is not yet well understood, and usually cannot be described by this model.
Another proposed mechanism is the DX center model, which involves photoexcitation of electrons from deep level traps which undergo a large lattice relaxation. According to this model, PPC is induced because recapture of electrons by DX centers is prevented by a thermal barrier at low temperatures. This model explains many PPC features as well as the large Stokes shift observed in Al.sub.x Ga.sub.1-x As materials. The nature of the DX centers is still being investigated intensively.
These two models predict the existence of PPC at low temperatures. Previously reported results of experimental investigations of most semiconductor materials have shown PPC effect predominantly at low temperatures. Neither, the macroscopic barrier nor the DX center models account for PPC which may be observable above 150.degree. K. and still exists up to room temperatures.
Previous III-V mixed crystal semiconductors, that is, semiconductor crystals formed of elements from groups III and V of the Periodic Table, exhibit PPC at low temperatures. However, PPC in these crystals cannot be quenched by infrared radiation.
The prior art also discloses solid state imagers in which an optical image is projected onto a large-scale integrated circuit device which detects the light of the image and develops corresponding image signals. A widely used imager of this type includes a charge-coupled-device (CCD) imager which absorbs light in a photo-responsive substrate, such as silicon. The absorbed photons of light generate charge carriers which are then used to develop the image signals.